The invention relates to cathodic arc plasma sources, and more particularly to sources adapted for efficient removal of macroparticles from cathodic arc plasmas.
Cathodic arc plasma deposition is a coating technology with great potential. Most importantly, cathodic arc plasmas are fully ionized and can therefore be manipulated with electric and magnetic fields. While electric fields are used to change the ion energy and thus the structure and properties of deposited films, magnetic fields are used to guide and homogenize the plasma.
However, a major obstacle to the broad application of cathodic arc plasma coating is the presence of macroparticles in the plasma, where macroparticles broadly encompasses all particles much larger than the ions, including droplets, microparticles, and nanoparticles.
Cathodic arc current is localized in minute nonstationary cathode spots. Spot formation is necessary to provide sufficient power density for plasma formation, electron emission, and current transport between the cathode and anode. Macroparticles originate from plasma-solid interaction at cathode spots.
Many approaches have been proposed and tested to eliminate macroparticles from cathodic vacuum arc plasmas. Most successful are curved magnetic filters, originally introduced by Aksenov and co-workers in the late 1970s. Although high-quality metal, metal-compound, and diamond-like carbon films have been synthesized by filtered cathodic arc deposition, macroparticle filters suffer from two major drawbacks: (1) the plasma transport is inefficient, i.e. only a fraction of the original (unfiltered) plasma is actually useable for film deposition, and (2) the removal of macroparticles is not complete. The latter is particularly pronounced for solid macroparticles as observed with cathodic arc carbon plasmas.
The design of macroparticle filters depends first and foremost on the mode of arc operation. DC arc plasma sources are usually equipped with cathodes of large size, e.g. diameter of 3-5 cm. The spot location may be magnetically controlled. In any case, the location(s) of plasma production, the micron-size cathode spot(s), can vary across the cathode surface, and the cross section of the filter entrance must be large enough to accommodate the various spot locations. A large filter entrance necessarily implies a large filter in length, volume, and weight. However, the plasma density in the filter drops exponentially with the path length of the filter.
Most filters, and virtually all of the DC-operated filters, have a xe2x80x9cclosedxe2x80x9d architecture in the sense that the filter volume is enclosed by a tube or duct which is surrounded by magnetic field coils. Macroparticles cannot leave the filter volume. They are expected to stick to the duct wall or to be caught between baffles that are placed inside the duct. The ducts are preferably bent, e.g. at 45xc2x0 or 90xc2x0, so there is no line-of-sight from the arc spot to the substrate.
U.S. Pat. No. 6,031,239 shows a filtered cathodic arc source with a filter of closed architecture having a toroidal duct with two bends, preferably in different planes, and a liner or baffle in the duct. The double bend provides no line-of-sight and no single bounce path through the duct. The duct is relatively large, with a diameter of 4-6 inches.
However, catching macroparticles is difficult for some cathode materials such as carbon because the macroparticles tend to be elastically reflected from surfaces. This xe2x80x9cbouncingxe2x80x9d problem is addressed by filters with open architecture where xe2x80x9cbouncingxe2x80x9d is used to let macroparticles escape from the region of plasma transport. Filters of open architecture do not have a duct but consist of a few turns of a magnetic field coil. The coil must have a relatively high current to generate sufficient field strength despite the small number of turns per length. For convenience, the arc current can be used in the filter coil.
Thus a short, open-architecture magnetic filter in combination with a compact arc source with a cathode of small area and operated in pulsed mode is desirable in order to have a high throughput of clean plasma to a deposition target.
Accordingly it is an object of the invention to provide an improved cathodic arc plasma source.
It is another object of the invention to provide a compact arc source with a cathode of small area and operated in pulsed mode.
It is also an object of the invention to provide a cathodic arc plasma source which can be used with a short, open architecture magnetic filter for the removal of macroparticles from cathodic arc plasmas.
It is a further object of the invention to provide a cathodic arc source for the deposition of thin amorphous carbon films.
The invention is a cathodic arc plasma source which has an anode formed of a plurality of spaced baffles which extend beyond the active cathode surface of the cathode. With the open baffle structure of the anode, most macroparticles pass through the gaps between the baffles and reflect off the baffles out of the plasma stream that enters a filter. Thus the anode not only has an electrical function but serves as a prefilter.
The cathode has a small diameter, e.g. a rod of about xc2xc inch (6.25 mm) diameter. Thus the plasma source output is well localized, even with cathode spot movement which is limited in area, so that it effectively couples into a miniaturized filter.
With a small area cathode, the material eroded from the cathode needs to be replaced to maintain plasma production. Therefore, the source includes a cathode advancement or feed mechanism coupled to cathode rod. The feed mechanism may simply be a manual feed mechanism, but preferably is a motorized feed mechanism automatically controlled by a controller.
The cathode also requires a cooling mechanism. The movable cathode rod is housed in a cooled metal shield or tube which serves as both a current conductor, thus reducing ohmic heat produced in the cathode, and as the heat sink for heat generated at or near the cathode. Cooling of the cathode housing tube is done by contact with coolant at a place remote from the active cathode surface.
The source is operated in pulsed mode at relatively high currents, about 1 kA. The high arc current can also be used to operate the magnetic filter. Power consumption and associated heat load can thus be regulated via the arc pulse duty cycle and pulse length rather than arc current which would lower the filter field.
A cathodic arc plasma deposition system using this source can be used for the deposition of ultrathin amorphous hard carbon (a-C) films for the magnetic storage industry.